Called "A Dry Run," the show will tell the story of "a group of American brigade members who come to fight in Spain against fascism during the civil war," a Mediapro spokeswoman said.
She added that the series -- planned as six one-hour episodes -- was still "at an early stage". On Twitter, Simon said the project still needed funding to go forward.
The 1936-39 war remains hugely sensitive in Spain, where the subject has long been swept under the carpet.
When dictator Francisco Franco, whose nationalists won the war, died in 1975 after decades of iron-fisted rule, authorities opted to draw a veil over the past for fear of further conflict as the nation transitioned to democracy.
As a result, this dark period of history is still largely suppressed, with Cambodia the only country to have more mass graves than Spain, according to Amnesty International.
Fiction has attempted to plug this hole and the war has been the subject of many films, novels and comic books.
After a Spaniard told Simon on Twitter that the war should "not be taken lightly", he responded that he was "aware that many in Spain would prefer no reflections on the past".
"Absolute national amnesia is the path preferred by some. But I trained as a journo, so no sale on that," he tweeted.
Have read all. Am aware that many in Spain would prefer no reflections on the past. Absolute national amnesia is the path preferred by some. But I trained as a journo, so no sale on that.— David Simon (@AoDespair) April 9, 2018
Mediapro has produced films such as "Midnight in Paris" and other Woody Allen movies, as well as television series including "The Young Pope" starring Jude Law.
The Spanish Civil War was “a dry run for the maelstrom to come,” Simon told Variety , indicating the origin of the series’ title. “But more than that, the events of 1936 to 1939 made clear that capitalism – while it may be an elemental tool for generating mass wealth – offers no moral answer to how people can live or how societies thrive.”
The series will be shot mainly in English, the language of the main characters.